Pull beards from mussels with a firm, quick motion. Scrub well and rinse mussels (or clams). Add a scant ¼ inch of water to 6 quart saucepan. Add mussels, scallops and shrimp. Rapidly bring to a boil. Reduce heat and steam for about 5 minutes, or until shells open. Drain. Discard any unopened mussels.